The present invention relates to non-oxide fiber reinforced ceramic matrix composites and more specifically to fiber debond coatings for use therein.
Continuous fiber reinforced ceramic matrix composites (CMCs) are being used and considered for a wide variety of components in advanced rocket propelled hypersonic missiles, reusable launch vehicles, air breathing turbine engines and a host of industrial applications. Candidate components include rocket thrusters and poppet valves of solid propulsion systems, rocket nozzle components and thermal protection systems for reusable launch vehicles, flameholders, combustors and divergent nozzle components for turbine engines. Currently, carbon fiber/silicon carbide (Cf/SiC), silicon carbide fiber/silicon carbide (SiCf/SiC) and silicon carbide fiber/carbon (SiCf/C) CMCs are the leading candidate materials for use in these applications. All of these CMCs depend on a fiber/matrix interface coating that is capable of transmitting the load from the matrix to the fibers as well as deflecting or blunting matrix cracks. The crack deflection capabilities are attributed to the interfacial slip or debonding of the interfacial coating from the fiber. The term debond coating is therefore commonly used. Therefore, the strength of the fiber debond coating to the fiber must be strong enough to transmit the matrix load but weak enough to debond from the fiber once cracks begin to propagate to the interface coating. Debond coatings must adhere to the underlying reinforcing fiber with enough strength to permit the fiber to provide its reinforcing function, while being able to xe2x80x9cdebondxe2x80x9d from the fiber and allow relative movement between the fiber and the coating during a stress event.
Another candidate system involves the use boron nitride layers coated or otherwise applied over a carbon, graphitic, layer as the debond coating material.
These debond coatings are applied to the continuous fiber reinforcing materials in fiber reinforced, ceramic matrix composites prior to the matrix densification step to improve the toughness and ultimate strength of the composite structure by, in effect, deflecting the energy of what could otherwise be catastrophic cracking events in the ceramic matrix of such composite materials when they are subjected to excess pressure, e.g. impact pressure, especially at high temperatures.
A major life-limiting factor for non-oxide CMC materials (carbon and silicon carbide materials) is the lack of a robust fiber-matrix interface coating for use in oxygen/oxidizing environments. Carbon and boron nitride are the commercial interface coatings for non-oxide ceramics currently commonly used. Both pyrocarbon and boron nitride (BN) coatings are not resistant to attack in oxidizing atmospheres, particularly at high temperatures. The pyrolytic carbon coatings currently universally used on both Cf/SiC and SiCf/SiC fibers are insufficient for long life components, particularly those that must operate in high temperature oxidizing environments. If stressed above the point where the matrix cracks, oxidants can penetrate into the fiber composite and oxidize the debond coating-matrix and debond coating-fiber interfaces. After very short periods of time, the composite""s mechanical properties deteriorate and eventually the material becomes brittle. Although boron nitride has demonstrated the potential to perform better than conventional carbon as an interface material in oxidizing environments, it too suffers from degradation in the presence of air and or moisture and therefore becomes life limiting. In fact, boron nitride can be characterized as a hygroscopic material. This makes boron nitride prone to failure in moisture containing environments such as are encountered in combustion applications.
Ti3SiC2 is a material that possesses high thermal conductivity, high electrical conductivity, reasonable hardness, good oxidation resistance, unprecedented thermal shock resistance, high temperature plasticity and room temperature machinability. Ti3SiC2 appears to be the high temperature equivalent of graphite for oxidizing applications because the planes of silicon are linked together by TiC octahedra in a hexagonal crystal structure arrangement. Like graphite, hot pressed Ti3SiC2 samples exhibit surprising yield strength at high temperatures (350 Mpa at 1300xc2x0 C.) and toughness (no degradation in strength after quenching from 1400xc2x0 C.). By comparison, the strength capabilities of the best superalloys are 200xc2x0 C. below 1300xc2x0 C. A detailed microhardness indentation study has shown that in Ti3SiC2 no indentation cracks were seen at loads as high as 300N. Scanning electron microscope analysis of areas surrounding the indentations revealed multiple energy absorbing mechanisms including diffuse microcracking and buckling individual grains. In non-oxidizing environments Ti3SiC2 retains its hexagonal planar crystal structure and high toughness/low shear characteristics at temperatures exceeding 2500xc2x0 F.
In brief, the wide temperature ductility, high thermal conductivity and oxidation resistance properties of Ti3SiC2 offers a fiber coating with unique energy absorbing capabilities that significantly reduces fiber cracking, oxidation and cyclic fatigue damage.
Similar results can be achieved in oxidizing environments by the application of single layer titanium suicides (Ti5Si3, TiSi2), multi-layer coatings of SiC/TiC and/or SiO2/TiO2 to the fibers, the latter being formed in situ.
It is therefore an object of the present invention to provide more oxidation resistant and therefore longer lived reinforcing fibers for non-oxide, ceramic matrix composites.
It is another object of the present invention to provide methods for the production of such enhanced fiber reinforcing materials and non-oxide ceramic reinforced composites fabricated therewith.
According to the present invention there are provided non-oxide debond coated reinforcing fibers that are resistant to oxidation at temperatures above about 1200xc2x0 C. The coatings of the present invention exhibit debond performance equal to or better than the prior art such coatings described above. The coated fibers of the present invention comprise a fiber overcoated with a non-hygroscopic, preferably silicon and titanium containing a single or multi-layer structure that imparts all of the properties demanded of a debond coating while additionally providing exceptional oxidation resistance protection. Single phase Ti3SiC2 coatings deposited onto carbon fibers, SiC fibers or other non-oxide fibers via chemical vapor deposition or otherwise form one of the embodiments of such coated fibers. According to a further preferred embodiment of the present invention, the coated fibers include an annular, thin, conventionally applied pyrolytic carbon layer between the fibers and the overcoated debond layer.
Multi-layer SiO2/TiO2 and SiC/TiC debond layers deposited by sol coating, chemical vapor infiltration or chemical vapor deposition processes or otherwise directly onto the fiber or over a thin conventionally applied carbon coating applied to the carbon, SiC or other non-oxide fibers optionally followed by oxidation of the deposited layer(s), form other embodiments of the improved debond coated fibers of the present invention. Single layers of titanium silicides (TixSiy wherein x=1 or 5 and y=1,2,3) and bilayers or multiple bilayers of SiC/TiC or SiO2/TiO2 can also be applied for useful results in oxidizing environments.